Carbon, filament, and method of making the same.



W. R. WHITNEY. l GARBoN, FILAMBNT, AND METHOD 0F MAKING THB SAME.

APPLICATION FILED FEB. 2, 1905.. l

Patented Mar. 30, 1909.

4 SHEETS-SHEET l.

INVENTOR Willie I? Winey,

W. R. WHITNEY.

CARBON, PILAMENT, AND METHOD OP MAKING THB SAME.

APPLICATION FILED PEB.2, 1905.

Patented Mar. 30, 1909.

4 SHEETS-SHEET 2.

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@l 6,905., f Patented Mar. 30, 1909. 6 sHEETS-SHEET 3.

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W. R. WHITNEY. CARBON, FILAMENT, AND METHOD 0F MAKING THE SAME.APPLICATION FILED PEB. 2, 1905.

916,905. l' Patented Mar.30,1909.

4 SHEETS-SHEET 4.

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Percentgges af vo/tgges ,gh/'13g /watlts per candle l l l0 2O 30 40 5060 70 8O 90 |00 IIO |20 |50 |40 Witnesses: v Inventor:

Willis' Rhitne,

UNITED STATES PATENT OFFICE.

WILLIS R. WHITNEY, OF GLEN VI-LLE, NEW YORK, ASSIGNOR TO GENERALELECTRIC COMPANY, A CORPORATION OF NEW YORK.

CARBON, FILAMENT, AND METHOD 0F MAKING Tm SAME.

' Specification of Letters Patent.

Patented March 30, 1909.

Continuation of applications Serial No. 192,586, led February 8, 1904,and Serial No. 198,866, filed February 15, 1904. This application ledFebruary 2, 1906. Serial No. 243,781.

To all whom it 'ma/y concern.:

Beit known that I, WILLis It. Wm'rNEr, a citizen of the United States,residing at Alglaus, in the town of Glenville, county of chenectady, andState of New York, have invented certain new and useful improvements inCarbons, Filaments Thereof, and Methods of Making the Same, of which thefollowing is a specification.

My invention or discovery resulted from an attempt-to improve carbonfilaments such as are used in incandescent lamps,'and embraces not onlya novel filament and a novel method of manufacture thereof, but also thesubstance involved in the composition of the filament, which substanceI' believe to be a new form of carbon, or more specifically a new formof graphite.

Before proceeding to a detailed description of my invention it will beof assistance to refer briefiy to the mode of manufacture of theordinary carbon filament, which is substantially the only lament in usein this country to-day. This process consists in dissolving good, clean,commercial cotton or other natural cellulose in some suitable solvent,and s uirting the same through a die into a fluid ardener. The resultinghiament is then dried, cut into lengths and care bonized by heating outof contact with the air atssuch temperature as may readily be obtainedby a gas furnace. The filament in this condition has long been known tothose skilled in the manufacture of incandescent lamps as a basefilament and consists of carbon in a form very like dense, hardcharcoal. It is usual to treat or ash the base filament after thecompletion of the process described above, by immersin it in ahydrocarbon vapor, such as vapor o benzin, and passing such a currentthrough it as to raise it to a temv erature which will cause adecomposition o the hydrocarbon and a conse uent deposition of theresulting carbon or car on compound lon the filament. This flashingprocessreduces the resistance of the filament, and in commercial work itis customary to thus flash the filament until its resistance reaches acertain standard value depending upon the "particular circuit on whichthe lamp is intended to run, the candle owerdesired, etc., and thisfiashing process 1s the last step in the manufacture of the filament. VThis coating or deposit from the hydrocarbon vapor consists, Vwithoutdoubt, principally of carbon, and tests which I have made show that itis in the form known as graphite; at least it answers the standard testsfor graphite as established b Berthelot, an eminent authority on this suject, as for example b producing graphitic acid, a yellow insolu lesubstance, when treated with highl 'oxidizing reagents, such as amixture of an ydrous nitric acid and potassium ohiorate, while ordinarycarbon is dissolved thereby, and further it gives the' characteristicgraphite luster when rubbed on paperf The ordinary commercialv carbonfilament conslightly impure, surrounded by a shell or coating consistingmainly or entirely of graphiticcarbon or graphite. Such a filament maybe said to be composite.

Characteristic features by which incandescent lamp filaments may berecognized and distin uished are their resistance curves and their ifecurves. The exact nature of these curves will be explained below, butfor the present it will be sufficient to state that a standard carbonfilament of the rior art, either flashed or unflashed, as a ovedescribed, falls in electrical resistance, when its temperature israised above atmospheric temperature, until near its standardtemsubstantially fifty per cent. of its cold resistance, while theuseful life of va lamp containing such a filament is somewhat less thanfive hundred hours when burning at such voltage that for every three andone-tenth watts of electrical energy imparted to the lamp one candleower of light is produced. l may here remar that the number of wattsexpended for each candle power of light is the usual measure ofefficiency of the amp; the fewer the watts per candle the higher theefficiency. Incandescent lamps fall in efficiency as they are used andit 1s obvious that a certain amount, that is to say, wheny a relativelylarge increase in the number of Watts necessary to produce each candlepower takes lace, it 1s no longer wise to burn that lamp but it is thenmore economical to destroy it and to re lace it with a new one. `-Theexact loss of e ciency which justifies the destruction of a lamp or, asit is cal-led in the art, the exact smashing point of a lamp,

sists then .of a dense, hard carbon base,

erature of incandescence its resistance is.

when a lamp has fallen in efficiency beyond must depend upon variouspractical conditions such as the cost of producing and supplyingelectric current, tie cost of the 1am itself, etc., so that the actualpractice of different lighting stations is n ot uniform, but it is(luite generally recognized that a lamp whic 1 has fallen to ei hty percent. of its initial candle power shou d be removed or destroyed; that1s to sai, eighty per cent. is somewhat arbitrarily, ut quite generally,taken as the smashing point. It will also be obvious that the usefullife of any incandescent lamp, or the number of hours of burnin beforethe smashing point is reache will depend upon the rate at which it isburned or, in technical language, the efficiency at which it is run. Byincreasing the amount of electrical energy supplied' to a lamp (byincreasing the voltage at the ter--` minals) it is possible to increasegreatly not only the amount of light given by the lamp but even theefficiency, but only at a great sacrifice of useful life. For example,if the ordinary incandescent lamp beburned at two watts per candle powerit will have a useful life of roughly forty hours or less, whereas ifburned at the usual efficiency of 3.1 watts per candle it would have hada useful life of a little under 500 hours.

My researches have led me to the discovery that if a fiashed filament,such as I have described above, be heated or fired at certain extremetemperatures which I havel been able to establish in a carbon tubeelectric furnace, the characteristics of the filament under o mostremarkable changes; thus thecol resistance of the filament after suchheatin is very much lower than the original col resistance, while theresistance at running temperatures is either not appreciably reduced oris reduced to a less extent. 'lhus the resistance of the ordinarytreated or flashed filament of the prior art,

-always becomes as low as a out as stated above, decreases rapidly withincrease of temperature; and at some point not far removed from therunnin tem erature ty per cent. of thecold resistance. But theresistance of the new filament never becomes, with increase intemperature, as low as fifty per' cent. of its cold resistance, and ineases where the firing in the furnace has been carried on at extremetemperature it actually becomes much higher than the cold resist- Themean "temperature-resistance coefficient, between 20 degrees centigradeand the temperature at which the lamp .is run, of the best filamentsmadein accordance with my invention is therefore positive, while thecorresponding coefficient of the ordinary filament of the prior art,fiashed or unflashed, is strongly negative. It will ybe understood thatthe beneficialeffects of myl invention may be abtainedi-v by firing ata, very high temperature within a region which ciaooo I have hereinafterattempted to indicate. If'the tcm erature of firing approaches theminlmum imit the resulting filament, as compared with an ordinaryflashed carbon filament, decreases a less percentage in resistance withincrease in temperature than does the ordinary filament when heated tothe same temperature. If the tempel-alum` of firing be considerablyhigher than the minimum limit, the filament, when liit, has a resistancehigher than its cold resistance. Results intermediate between these areobtained by firing at temperatures between 'the extremes mentioned, andI'desire it to be understood that although I prefer to use temperaturesthe highest conveniently obtainable, my invention is to be construed ascovering filaments herein indicated. p

The characteristics above discussed serve to hel to identify the articleconstructed in accor ance with m present invention, but from the pointof vlew of incandescent-lamp manufacture the most important feature ofimprovement lies in a certain increase which my present invention makesin the efficiency or life of the lamp; that is to say, I find that whenthe lamp in which my filament is placed is run at such a candle-powerthat its useful life is reduced to that of the ordinary carbonincandescent lamp above described, an improved efficiency is obtained,while if the amp is so run as to give the same efficiency appear inefficiency or in life or in a combination of both.

' I will now describe more'in detail the best manner known to me ofcarrying out my invention and will do so [in the knowledge that theinvention is susceptible of many modifications.

'Ihe scope of my invention I will indicate in the claims appendedhereto.

In the drawings attached hereto, Figure l is a view in section of afurnace such as I use; Fig. 2 is a sectional View of a carbon capsule orboat; Fig. 3 is a perspective view of 011e of the plugs'ftting into theends of the capsule or boat; Fig. 4 is a set of curves showing thevariations of ohmic resistances of a number of filaments with variationsof applied voltages; Fig. 5 represents the variations of the samefilaments but plotted so as to show the 'Ihe plug thus serves as amechanical support 4tion tting into the respective ends. @ne of r theplug as indicated by the direction line at stages in my process ofmanufacture; and] Fig. 8 represents resistance characteristics ofcarbonized bamboo filament subjected to my process. i

In Fig. 1, A is a base or foundation and B B are terminal walls ofbrick, constituting with i the side walls a brick chamber open at the itop. The ends of the carbon tube C are clam ed into water-cooled,carbon-hushed, meta terminals D having openings E, concentric with-thebore of the tube, through which material to be fired is passed. Waterconduits F and current-conveying leads G are fixed to these terminals,while the whole space about t'he tube is filled with powdered carbon orraphite as at H, either with or without a ayer of titanium carbidimmediately surrounding the tube. l usually close the ends of the carbontube by plugs of asbestos wool. It is evident that the carbon tubefurnace may be of any suitable construction, but ll have obtained verygood results by using a furnace such as represented in the drawings, inwhich the carbon tube is about two and one-half inches Iin outsidediameter, one and threequarters inches inside diameter, and twentyinches in total length, including the taper at both ends.

ln conducting the firing o eration, the filaments, before being inserteinto the tube of the furnace, are inclosed in suitable boats orcapsules. One form" of boat which il have used consists. of a tubefilled with powdered graphite held in place b removable Stoppers. '.lhefilaments, in bunches, 4are placedm the tube and the graphite powderpacked around them. Instead of this simple form of capsule l may use theform shown in Fig. 2 of the drawings, in which the carbon tube l isnearl filled by Stoppers of special constructhese sto pers is shown inperspective in Fig. 3 and3 consists in general form of a cylinderprovided with longitudinal grooves to receive a bunch of coiledfilaments. One leg of this bunch extends along an under-groove in K;then it passes over the end of the plug at L and after looping down andback through the cut M, which extends completely across. the plug, turnsand lies along the groove N.

for the bunch of coiled or looped filaments. Plugs of this characterinserted in the respective ends of the tube I, Fig. 2, carry the bunchesof filaments as indicated at O and F. Before proceeding to actual firingof the filaments, the furnace, together with the boats and any packingmaterial such as raphite which may be used therein, is subjectedA to apreliminary firing in order to drive off impurities commonly present inthe materials used. For this purpose the furnace, with the boats inplace in the tube, but in this case without the filaments, is heated tosuch a temperature as to drive out inost of the mineral impurities suchas clay,l silica andthe like. If this reliminary firing be not employedthe resu ts obtained are apt to be unsatisfacto batch or batches ofilaments fired.

The furnace and the boats havin received their preliminary firing, theoperation of the firing of the filaments may then be cornmenced l l findit desirable for reasons described below, though not essential, tosubject the base filament to which my invention is to be applied to areliminary firing before it is treated or flas ied,v A

My processthen in its more perfect form consists in firing an ordinarybase filament at a very high temperature, allowing it to cool, thensubjecting it to the ordinary flashing or treating process, whereby itreceives a deposit of what l believe to be 'graphitic carbon, and thenagain firing it at a very high temperature.

The current which I use with the furnace having a tube of the abovedimensions may vary from about 2000 amperes to about 3000 amperes. Theenergy employed may vary; l et good results with the expenditure of 33ki owatts. The temperature of a furnace of given construction anddimensions obviously de ends on the current, and owing to the difficu tyof measuring with accuracy the high temperature employed l sometimesfind it preferable to indicate, and -herein have indicated, thetemperature by reference to the current used for a known or standardvfurnace such as described rather than to atmary of a transformer,having a ratio of' turns of eighty to one, which l employ to supplycurrent to my furnace.

The time of passage of the filament throu h the furnaceduring firing maybe varier? widely. ln some cases l pass the boatsirather quickly throughthe furnace at such a rate that each boat is in the furnace only for afew minutes, and in other cases I have passed them more slowly so thatthe passage of a single boat through the furnace occupies several hours.l am of the o inion, however, that it is only necessary t at all thefilaments reach the proper temperature. I have also obtained goodresults simply by packing the. filaments in the boats with caron orgraphite dust, placing one or more of these boats in the middle of thefurnace tube, and supplying current at such a rate and for such a timeas practically to destroy the tube, though this last modification l haveused more commonly in the second firing of the filaments, after theyhave been flashed, than in the first firing. The temperature of thesecond fixing may or may not at least as to the first l shall t ereforebthe furnace, and then varying the voltage impressed on the lamp untilthe filament reached such a temperature that it could not be seenagainst the bright background of the furnace interior. After makmg thisadjustment it is found that with the furnace which I have described thistest lamp is running at about double its normal voltage or higher. Thus,if a fifty volt incandescent lamp be employed, then for the pur oses ofthis measurement, the lamp is foun to run in the neighborhood of onehundred volts or more. I have calculated this temperature from the lawof Lummer, that at high temperatures the ratio of luininosities of afilament equals the twelfth power of the ratio of the absolute tcmieratures. Assuming that the filament of the incandescent lamp has thesame temperature as the interior of the furnace, the temperatures whichI believe best calculated to produce the desired results appear to be inthe range between about 230() and 3700 degrees centigradeor higher.'.lhese temperatures, as measured in the above manner, check fairly withthe readings of temperatures of the furnace obtained by the use of aninstrument known to physicists as the Wanner spectre-photometricpyrometer.

It is difficult to state with certainty the minimum limit of furnacetemperature at which the beneficial effects of my invention areobtained, though I at present believe the results cannot be obtainedbelow 2000. degrees centigrade. Better results require a highertemperature, and for the best results the temperature must be at orabove a temperature much higher than this and at which quartz, platinum,and rare oxids, etc., vaporize and disappear. At such temperatures eventhe carbon tube is in a comparatively short time destroyed and, at thehighest temperatures mentioned above, the tube lasts only a few minutes.

The filaments resulting from this process possess the followingcharacteristics and may )e recognized thereby: The curve representingthe change o f resistance with temperature of the filament when mountedin a lamp is greatly modified as compared with that of the ordinaryflashed or treated filament, and in a degree 'depending upon thetemperature at which the filament, with its flashed coating, is fired.1f thiS.ll ifQg is done at a temperature in the lower portion of therange above indicated, the temperature resistance curve, with increasein temperature of the filament, -shows va minimum resistance greaterthan fifty per cent. of the cold resistance, while if the temperatureof' firing of this flashed filament be somewhat higher the curve dropsat its lowest point, or point of inflection, to say eighty er cent. ofthe cold resistance and then part y recovers, and at a still highertemperature of firing the curve is nearly a straight horizontal line,and at still higher temperatures the curve rises almost or quitecontinuously above the ordinate representing the cold resistance. Inother words, when the firing is carried on at extreme temperatures asbefore mentioned, the temperature resistance coefficient of. thefilament, except possibly, in a limited region near the starting pointof the curve, is positive., It is difficult to be certain that theslight dip near the starting point, shown in all the curves, reallyexists 1n the filaments fired at the higher temperatures. The bestfilament has its specific resistance at ordinary atmospheric temperaturereduced by the final firing process to a fraction, lsometimes as low astwenty per cent., of the resistance before,firing,`and its temperatureresistance coefficient becomes positive at least throughout the range inwhich the filament is luminous.' The-filaments in which the best resultsof my invention are obtained therefore have a higher resistance whenrunning in the lamp than when cold. These characteristics are obviouslysharpl distinguished from the characteristics of al other forms of'carbon.

I do not know of any way in which filaments made in accordance with myinvention may be pictorially represented by ordinary draftsmens methodsin such a manner as to distinguish them from filaments of the priorart.y I may, however, illustrate the characteristics of filaments madein accordance with my invention by the use of curves representing thevariations in magnitude of certain hereinbefore mentioned qualities ofthe filaments, and in the accompanying drawings I have provided a set ofcurves of this nature selected from a number of typical specimens whichhave undergone different variations of my rocess. All of the filamentsshown were brought to the same cold resistance "by the deposition of thecoating 4of the flashing recess and they were all as nearly aspossillile of the same dimensions, etc. In Fig. 4 the ordinates areplotted in ohms. The abscissae correspond to voltages, and in order toconform to a standard for the purpose of comparison are plotted inercentages of voltages which, when applic to the respective filaments,cause the filaments to. o erate atan efficiency of 3.1 watts per can le;-Thus while the voltages which give this efficiency may bev diEerent forthe sevand after flashing at 30 amperes.

the-filament of curve 3.

erases flashed and unfired; Curve 2 represents a similar resistancecurve of such a base filament after it has been flashed or treated andcorresponds to a filament of the best quality now in every day use thatis to say,l to a filament of the best standard commercial lamp'lofto-day. The remaining curves of this figure represent the observedeffects of firing such flashed or treated filaments, in accordance withmy invention, one at one temperature, another at another temperature,and so on. Curve 3 corresponds to a filament like that'indicated bycurve 2 but fired prior to flashing and again fired subsequently toflashing. The primary current supplied to the furnace (of the dimensionsabove set forth) during the first firing was 20 amperes, and that duringthe second firing was 26 amperes. lt will be noted that the initial orcold resistance of the filament is much less than that correspondingP tothe filament of curve 2. Curve 4 is that of a filament fired beforeflashing at 24 amperes, lt will be observed that with this highertemperature of the final firing the initial or cold resistance of thefilament is reduced more than that of Curve 5 corresponds to a filamentfired before ashing at 24 amperes, and after flashing or treating ,at 34amperes. The initial or cold resistance is thus still further reduced.Curve 6 corresponde to a filament fired before flashing at 34 amperes,and fired after' flashing at 34 amperes. Curve 7 representsv a filamentfired at 38 amperes before flashing, and again fired at 38 am eres afterflashing. Curve 8 represents a lament which was not fired prior toflashing, but which after flashing was fired at 26 amperes. Curve 9correspends to a filament fired at 28amperes before flashing, and thenfired after flashing at 38 amperes. ment which was given a firing at 24amper-es prior to flashing, and was then fired at 38 amperes afterflashing. In general'ithe hotter the finalfiring the lower seems to bethe cold. resistance. Certain apparent discrepancies in this respectwill be discussed hereafter. 4 i

For showing in a general way the changes produced in the resistancecharacteristics of filaments by firing, reference may no w be made tothe changes in the character of the resistance curves corresponding tothe respecpera ture.

Curve l() corresponds to a fila-- tive fired filaments. Thisv comparisonis easily made, even when the filaments have different cold resistances,by replotting the curves so that the ordinates represent ercentages ofthe cold resistances rather t an absolute values, which were shown inFig. 4, bv which means/the curves are made to start all from the samepoint, as shown in Fig. 5, and relative values are vcompared rather thanabsolute values. The curves in this figure are numbered from l to l0 andcor. respond respectively to the similarly numbered curves in llig. 4.lt will be seen that in a eneral way the filaments which had their nalfirings at the lowest tem ieratures, and still more that filament 2 whici was not fired at all, decrease most in resistance as theirtemperaturey increases, while those filaments fired at highertemperatures decrease in resistance .less or actually increase inresistance as compared with 4their cold resistance. In all of the casesplotted the resistance curves drop at first and then either becomenearly horizontal or actuall rise. 'lhus in the case of curve 10 theresistance rises practically `continuously above the cold resistance, sothat the temperature coicient may be said to be positive throughpractically the entire range. lt will be o served moreover that in curve2, which corresponds to an ordinary commercial filament, the resistancewith increase of temperature drops to as low as fifty per cent. of thecold resistance, whereas in the case of all the fired filaments the dropin resistance is much less and insome cases there is, instead of a drop,an actual and very considerable rise in resistance -over a considerablerange of tem- Thus in the case, for example, of,

lament 10 the resistance .at the running temperature at which the lampgives 3.1

`watts per candle is about forty per cent.

hi 'her than when the filament is cold.

n order to perceive the effect produced upon the life offilamentsprepared in accordance with my invention or, in other words, to see howmy invention affects the action of an incandescent lamp, reference maybe made -to Fig, 6.- ln this ligure the abscissae of the curves areplotted in hours of burning and the ordinates in percentages of candlepower. 'All of the tests from which these curves were made were madeunder the same `conditions and were reduced to an efficiency of 3.1watts per mean horizontal candle power, which is at present regarded asa high standard efficiency and is the rating env which lar e numbers ofthe best lamps are sold. he curves in Fig. 6 are numbered from l to 10as in thepreceding figures and correspond respectively therewith. Curve2, Fig. 6, represents the variation of candle power with hours ofburning at 3.1 watts per candle of the ordinary good commercial filamentor, more accurately,

of a lam containing such a filament. It will be o served that at the endof a little less than 500 hours this filament falls to eighty per cent.of its initial candle power. The improvement produced by ilaslung ortreating is seen by com arison with curve 1 representing the life o thecorres ondlng base filament. The other curves o Fig. `6, representing'filaments fired 'under varying conditions and temperatures, showl areat increase in life over that of the ordlnary filament. Thus curve 3,representing a filament fired atthe lowest of the temperatures which lhave used, shows a useful life at 3.1 watts` per candle of somethinglike 680 hours. of the otherflaments may be observed. The lamp of curve6 for example did not fall to eighty per cent. until the expiration ofabout 2000 hours, that is to say its useful life was over four timesthat of a standard lamp. Some of my results are still better, the usefullife of some filaments having passed 2500 hours at an efficiencycorresponding to 3.1 watts per candle, corresponding to about 720 hoursuseful life at 2.5 watts per candle. There is not a strict agreementbetween all the curves, yet as a general rule they show that the higherthe temperature of firing the longer the useful life. lt must be bornein. mind, of course, that the results obtained are liable to variationdepending upon conditions under whicjhf'the rocess is carried out, oreven upon inevitab e variation in the filament or the-treatment. Withany articular set of conditions, however, suc as may be du' licated inpractice, -the results obtained su stantially agree.

ln the foregoing sets of curves l have represented the comparativeeffects of differ-r ent temperatures of firing, but without tracing thechanges which a single filament under- The general nature of goes in theprocess. these changes l have roughly indicated in Fig. 7. ln thisfigure the curve 11 represfnts the variation of resistance withtemplerature of an untreated or uniiashed base ament. After this basefilament has received its first firing, in accordance with my invention,the temperature resistance curve becomes changed as represented at 12.It will be seen that the cold resistance is practically unchangedwhilethe hot'resistance has i become slightly lower. Upon iiashing ortreating this base filament in the usual manner in a hydrocarbon va orthe resistance, of course, is greatly reduce though the charac- Y ter ofthe curve of resistance may not be very tion of curve `13. Upon firingsuch a las red greatly altered, as' will be seen by an ins )ecfilamentin accordance with m invention,

and in this instance ata relative y hi h temperature, alterations inthefilament ta e place such that the temperature resistance curve ischanged to that indicated at 14. This last In a similar way the lifecurves curve shows that the firing lowers the cold resistance but maynotgreatly change the hot resistance of the filament. Obviously thefilament considered in Fig. 7 is a compositefilament, that is to say, itis a filament consisting really of two conductors in parallel, an innercarbonbase and an outer shell or skin. 1f thc curve 12 is the resistancecurve of the fired carbon base before flashing, the curve of the carbonbase after the second firing will be similar in form since the secondfiring makes, as lhave observed, relatively little change in the natureof the base. The curve 14, being thc curve of the finished filament,obviously represents the temperature resistance characteristics of theabove-mentioned composite conductor, and by' calculations based on thewell-known mathematical laws of parallel electric circuits it ispossible to calculate the curve 15, which is the curve of thetemperature. resistance characteristics of the skin or coating. Thiscurve 15, the' temperature resistance curve of the skin or coating of myl -fore that some profound change has taken placein the natureof thisskin or coating. l have found that it is possible in some cases toremove from a filament produced inaccordance with my invention, forexample from an ordinary sixteen-candlepower filament `which has beensubjected to the treatment above setforth, a section of the shell, coat.ing or flashing. By careful mani'i'mlation l. have obtained suchsections an inch or more in length and find such a section to consist ofa little tube with very thin walls. The nia-- terial of this tube.possesses characteristics, as above indicated, radically dille-rentfrom those heretofore predicated of carbon. 'I believethis tube to be anew form of carbon,

and ispecifically a new form of graphite, since it di rs from ordinarygraphite or from the original graphitic coating ofthe filament in manyrespects. It lanswers the chemical test for graphite abovel set forthbut reacts with" oxidizing reagents (such asA nitricacid and potassiumchlorate) to form gaphitic acid very much more readily than does anyother form of graphite with which I am ac'- quainted. For example thisnew form of graphite was treated for several hours at 60 degrees C. in amixture of anhydrous nitric acid and potassium chlorate. It was thenfiltered, washed with nitric acid and the above operation was repeateduntil it had been performed four times. The powder was then treated withotassium permanganate and finallywith iydrogen peroxid. The sameoperation was performed the same numb er of times with a material whichis commercially know as Acheson graphite, an artificial raphitevproduced, by exposing carbon to hig temperature'. At the en d of thisprocess the Acheson graphite was dark green while the new form ofgraphite was yellow or yellow-green showing that it had been morecompletely transformed. The properties of this new form of graphite mayvary de )ending upon the temperature of the final iring. lf thetemperature of firing be such as corresponds to curve 3, Fig. 6, or inthat neighborhood, the temperature resistance curve ol' this new form ofgraphite may be such as to show a slight negative temperatureco-efficient; that is, it may depart to a certain extent from curve. 15,Fig. 7, in the direction of curve 16, Fig. 7.

Throughout this specification where il have alluded to temperaturecoefficients l have been considering these coefficients between thetemperature of 20 degrees centigrade,. that is to say, the ordinarytemperature of the air, and the temperature of working incandescence ofthe filament.

In general this new form of `carbon is characterized by a temperaturecoefficient either actually positive or if negative less in amount thanthat of any other form of carbon known, and it may, if the tem eratureof the final firing. has been high, ave a positive temperaturecoefficient such that its resistance at running tem eratures is verymuch in excess of its col resistance. Such a temperature characteristic,that is to say, a temperature coefficient such as to give in all cases atemperature-resistance curve rising substantially above thecharacteristic temperature-resistance curve of carbon, as explainedabove, l call herein a metallic temperature characteristic. Anotherimportant characteristic of this new material is its low resistanceatordinary atmospheric temperatures. This characteristic again may varydepending u on the temperature of the final firing and) also indirectly,as explained below, on the temperature of the preliminary firing, but inany case it is relatively low. In filaments which I have produced thespecific resistance at ordinary temperature, say 20 degrees centigrade,of this new material varies from 0.00006 to 0.00016 ohms per cubiccentimeter; or 0.6 to 1.6 ohms per unit of one meter long by a squaremillimeter section. Zellner (Die Kmstlchen, Kohlen, 251, JuliusSpringer, Berlin 1903 gives the resistance of ordinary carbon per unitof'one meter long by a square millimeter section, as 20 to 50, Ceylongra hite 2 to 8, and Acheson artificial grap ite 12. The material isnon-magnetic, and may have a decided luster. its specific gravity isconsiderably higher than that of .the original ublished by flashing orcoating before the-heat treatment.

The tube above referred to is believed toconsist entirely of the newform of carbon above mentioned, and constitutes a hollow filament whichis capable of bein mounted and operated as the filament o a lamp. wing,however, to the difficulty of' obtaining these tubes in lengths suitab efor lamps of the more common standard voltages, l find it better, at thepresent time, to use the composite filaments consistin of a core ofordinary carbon and a shell o the new form of carbonas fully describedabove;

My invention may be practiced and iinprove dresults obtained, by a'single ring, at the proper temperature, of vthe ordinary flashed ortreated filament. lin describing, however, the best mode known to? me ofcarrying out m invention, l have prescribed a firing of the ase filamentprior to flashing or treating thereof and a subsequent firing after theflashing or coating obtained by treatment in the hydrocarbon has beenapplied.A

This double firing .obviates or prevents the alppearancev of minutebubbles or blisters on t ie surface of the filament as mayoccur,particularly if the filial firing is at verily high temieratures,when no rehniinary ring of the iiase filament is ha These bubbles havethe a pearance of beinglcaused by gas pressure i eveloped within t efilament while the coating orflashing is in a asty condition. lt isprobable that the bubb es are caused by the eHort of impurities,metallic or otherwise, to escape from the base filament through thecoating or flashing. The preliminary firing of the base filament, byremoving to a greater or less extent these im:`

purities, has, l find, the effect ofpreventing the appearance of thesebubbles in the filanient. The double firing of the filaments results ina somewhat different ap earance the completed article. 1When t efilament is fired after flashing, but not before flashing, the surfaceis black and shiny althoughapt to be marred by the presence of bubblesor blisters. twice, once before flashing or treating and once after,will have a surface which is free from bubbles and which may, undercertain conditions, have a ray, velvety appearance. I think it best toraise the temperature of the lament rather slowl in the first firing, to

`avoid a weakening o its structure due to a ice y A filament which hasbeen fired the furnace at the high temperatures hereinbefore mentioned,I can obtain results similar to those characteristic of the filament Habove described. In Fig. 8 the curve 17 is the temperature-resistancepercentage curve, before firing, ofsuch a filament Ihaving a very thincoating, while curves 18 and 19 show the changes in resistance producedby firing,

- curve 19 corresponding to u higher tmnperature of firmg than curve1.8.

Having thus described my invention and.

the best manner in which it can be carried out as known to me at thepresent time, I do not rest my right to a patent on any theory which Ihave formed of its operation, but will nevertheless now proceed to statewhat I surmise to be the action occurring.

I am at present of the opinion that certain impurities normally existingin the base fila- 20 ment, probably lthe potassium, on being driven outby the heat ofthe electric furnace, unite with the graphite coating orflashing and enable it to soften and'sinter together in some'combination which is at present unknown to me, perha s a sim lepotassium carbon solution, an that t iis impurity, whatever it may be,is subsequently driven ofi', leaving the particles of the resultinggraphite or carbon associated with each other in a relation differentfrom the relation in which they are associated in all yforms of carbonhitherto known to me and probably more analogous to the manner ofassociation of the particles in ordinary metals. Whether this theoryybecorrect or not the invention results in the production, first, of anentirely new form of carbon having valuable uses in the arts andparticularly useful in an incandescent lamp filament; and, second, in anincandescent lam filament of high eflicieney and long life. his theorywould tend to explain certain of the curves above mentioned, for examplecurve 7 shows a filament which was fired at 38 amperes before flashingand 38 amperes after flashing; and curve 8 shows a filament which wasnot fire-d before flashing but which after flashing was fired at 26amperes; while curve 10 corresponds to a filament which was fired at 24amperes prior to flashing and then fired at 38 amperes after flashing. Acomparison of these curves would indicate that a high heat in the pre.

liminary firingtended to reduce the drop of resistance. which resultsfrom the final firing, which would be reasonable on the aboveassumptionsince an extremely high tenfiera ture at the first firingtends to drive o the impurities which are supposed to play a part in theprocess carried on during the second or final firing. So also this mayhelp to explain the fact that the filaments which have been fired to ahigh temperature after flashing but which have not been fired or whichhave been fired at a relatively low temperature before flashing have avery smooth, black, l fact whic ample salts of potassium or even saltsof lustrous coating, while filaments otherwise the same, but given apreliminary high temperature firing before flashing, are not so smoothorlustrous but seem to have aslightly grayish tinge as if the materialhad not run together as freel As a confirmation I may mention that havefoundA that the introduction of. certain impurities, as for exlea intothe .carbon boats in which the filaments are exposed to the action ofthe electric furnace, has atendency to cause the coatings of thefilaments to have a very smooth, black luste^r and even to run togetheror bead It is evident that this action, if this theory is correct,cannot take place .while the filament is employed in an ordinaryincandescent' lamp, since it would depend upon the fact that theimpurities, potassium or whatever they maybe, have, 1n the. furnace, acertain vapor tension or at least remain near the filament, while in avacuum they would be very much attenuated `and driven ofi. Thiscorresponds' to the fact well known in chemistry that the chemicalaction between two substances in contact ldepends largely on theirrespective vapor tens-lons or concentrations. It is, of course, wellknown that the filament of an incandescent lamp is not improved, butrather injured, by subjecting 1t to abnormally high temperatures duringloperation, as by abnormally increasing t e voltage. For example, thetest lamps mentioned above 'are destroyed in a few minutes by the highvoltages used in the tests. There is therefore reason to believe thatthe best results are to be expected when the filaments are massed inbunches and placedin small carbon receptacles as described and exposedto high temperatures at a Ipressure at least equal to the orderofmagnitude of atmospheric pressure. I have also found thus far, thoughthis conclusion is not final, that better results are' obtained by'relying u pon 110 the natural impurities suchas are foun in ordinarycommercial pure. cotton than b ialtterlipting to introduce theseimpurities artlcia While I have thus set forth my hypothesis 115 as tothe nature of the action occurring when my invention is practiced, it isnevertheless true that others skilled in the art who have followed theprogress of the work have advanced the hypothesis that the coating "or120 flashing on an ordinary commercial filament may be a compoundsubstance such for example as a hydrocarbon containing an ali-- normallylow percentage of h drogen, or, in other words,a very high hy rocarbon,and 125 that the efi'ect of the final firing is to change the nature ofthis hydrocarbon, producingr erhaps a still higher and heretofore unnownhyldrocarbon. One, and the only,

seems to me to favor this last 13o perature in the preliminary heatingor firing second,

erases hypothesis is that hitherto the best filaments made in accordancewith my invention have been, broadly speaking: those which have beenheated to the hig est temperatures in both of the heat treatments andwhich therefore have been heated to such a high temthat it is notunreasonable to suppose that all of the impurities were eliminated. Iam, however, of the opinion that the first firing even at the extremetemperatures mentioned does not ,entirely eliminate the impurities, andfurther'that itis quite possible that enough potassium .vapor remains inthe furnace from the first treatment, or in the' carbon boats, toproduce the results above described. Careful tests ina/de under my`direction have not shown the existence of hydrogen inthe coating orflashinfr of 'fila- ,ments to which my invention has been applied. Thediscovery hereinbefore referred to, that if the firing before flashingis at a lower temperature than the iirin Aafter flashing the 'lilamentsbecome smoo `er,.. blacker and more lustrous than if the first iiring isat substantially the same temperature as the was made subsequent-to myinvention by one John W. Howell and was utilized by himfas amodification of or as, an im- What I claim as newand desire to secure byLetters Patent of the United States, is,-

1. An artificial graphite lhaving a metallic temperature-resistance characteristic. l

'2. An artificial gra hite of specific resistance at 20 degrees of lessthan 0.00016 ohms per cubic centimeter.

3. A'4 carbonaceous substance having a specific resistance, at 20degrees centigrade, o not .over 0.00016 ohms per cubic centimeter. l i

4. A graphitic substance having a teme perature resistance percentagecurve running above that of ordinary graphite. v

5; A carbonaceous substance characterized b a resistance at atmospherictempera- `tures elow that of other forms of carbon of .the samedimensions. and having its resist-` ance at about the temperature of..incandescence ap roximating more closely'to the resistance o otherforms of carbon at about the temperature of incandescence.

A n artificial raphite of low specic reslstanceand whic has a higherresistance when hot than at atmospheric temperature.

7. Acarbonaceous substance having ametallic luster and a low specificresistance, the resistance of which when hot is higher than itsresistance at ordinary atmospheric'temperatures. y p

8. A form of graphite-Which, as'compared withl other known graphites,reacts more uickly with a mixture of nitric acid and c orate of potashto iorm graphitic acid.

9. A lament for an incandescent lamp consisting of a carbon base, and acoating thereon deposited by decomposition, of. a hydrocarbon, whichcoating has been altered in character subsequent to its deposition onthe base. l

10.` An incandescent lamp filament coated with carbon, the resistance ofwhich is greater when the ilament is hot than when at ordinaryyatmospheric temperature.

11. A carbonaceous lament, the resistance of which upon increase'intemperature in no case falls as low as approximately sixty per cent. ofthe resistance when cold.

12. A carbonaceous filament, the resistance of which upon increase inYtemperature may increase greatly above the resistance at ordinaryatmospheric temperatures, and in no case falls as low as approximatelysixty vper cent. of the resistance when cold.4

13. A filamentl of carbonaceous conducting material which, as comparedwith the filaments of' such material heretofore known, is much lower inresistance when eold and, when heated to o erating temperature, iscomparatively litt e less in resistance.

14. A carbonaceous filament having its resistance when cold below thatof a standard -c-arbon filament of the same dimensions and its`resistarwice when heated to the running temperature'considerably nearerto that of an ordinary ilament under similar conditions.

15. A carbonaceous filament whose resistance curve from cold to runningtemperature is more nearlyr flat than the corresponding ciirve of astandard'carbon ilament. 16. A filament containing carbon which istough, flexible and of low specific resistance at ordinary temperaturesand Which -has a temperature resistance percentage curve running abovethoseof other graphites.

- 1'7. A filament containing carbon, the rei isistance of which, withrising temperature,

increases substantially continuously over and above the resistance whichit has in the neighborhood of atmospheric temperature. 18. A filament ofcarbonaceous'm'aterial the temperature-resistance curve of which withincrease of temperature from 20 degrees C. drops' in resistance butslightly and then rises a ove the initial point. 19. A compositefilament, the resistance of which, upon increase in temperature, remainsabove approximatelyeiifty per cent. of the resistance when cold.

20. A composite filament consisti 'of a base oi carbon, and acarbonaceous s coating, which skin Ior coating has 'a greatervresistance when hot than When-cool. 21. A composite lament consistingoi a base of carbon, and a skin or coating of a iis form of carbon,which skin or coating has a thereon, the 'oint resistance of Which isgreater when the ement `is hot than when at ordinary atmospherictemperature.'

23. rlhe method which consists in deriving a carbonaceous productthrough decomposition of a hydrocarbon, and then heating or tiring theproduct, in a non-oxidizing1 atmos-` phere, to a tem erature far highert an that necessa to e. ect such decomposition.

24. T e method which consists in depositing a carbonaceous productthrough decomposition of a hydrocarbon, and then heating or firing theproduct, in a non-oxidizing atmosphere, .to a tem erature far higherthan that necessary to e ect such decomposition and high enough so thatthe minimum resistances of the product, before and after firing,occurring in a range of rising temperature, as'compared with therespective cold resistances, forms a higher percentage in the case ofthe fired product than inthe case of the unred product.

25. The method 'which consists in ,ex )os- .graphitic carhonaceousmaterial suc i as is deposited from a h drocarhon to a very hightemperature, as erein specified, in an atmosherecontainin potassium. 26.i method cfg improving flashed or treated lanients which consists inheating or g them, in a non-oxidizing atmosphere, to a temperature verymuch higher than that employed during'flashing and high enough so thatthe minimum resistances of the tired and v unred filaments respectively,occurring in a range of increasing temperatures, as com- Q ared'withtheir'res eetivercold resistances,

Orme-'a'. vpermanent y higher percentage in the Gasey of the firedilament thanin the case of ,the untired lar'nent.

" 27. The method which consists in heating a flashed or composite lamentto an eX- tremely high temperature in a carbon electric furnace.

28. The method which consists in heating erases a dashed filament byexternally-ap lied heat to a temperature and for a length o timesufldicient to produce as a product a lament, the

resistance of which, with increase in temperature, becomes greater thanits cold resistance.

'29. 'lhe method which consists in forming a base. filament, depositingthereon a carbonaceous coating, and then heating or firing the filamentthus coated 'to a temperature suflicient to render positive thetemperature-re- Sistance (eo-efficient of the lament.

30. The method which consists in firing a base" filament at not lessthanv 2000 degrees centigrade, flashing or treating, and then againfiring at a temperature no t less than 2000 degrees centigrade.

31. The method which consists in heating an prdinary untreated orun'lashed carbon filament to a teni )erature substantially higher ,thanthe car onizing temperature,

then flashing and then tiring to a degree and under conditions sulicientto increase greatlv the length of life of the filament.

" 32. 'lhe methodhwhich consists in heating an ordinary untreatedorunflashed carbon filament to-v a high temperature suilicient to driveoutimpurities, t

ment, and then firing the tia-shed filament to a sufficient de ee tochange permanently the resistance c aracteristics of the filament.

33. The method-Which consists in gra-dually heating up a base ilament toa temperature sufficient to drive' out im )u1-ities, then flashing thefilament, and then leal ing orliring the flashed filament to arlegrecsu'lii'cient to produce permanent changes in its resist ancecharacteristics. v

In Witness whereof, I have hereunto set my hand this 31st day ofJanuary', 1905. z 'Ii y S R. WHITNEY.

i Witnesses: 4

BENJAMIN B. HULL,

HELEN Osman.

en flashing the -fila-i sol

